The present invention relates to photoacoustic or optoacoustic spectroscopy, and more particularly to applications that involve measurement of two or more gases or vapors in a mixture.
Photoacoustic measurement is based on the tendency of molecules, when exposed to certain frequencies of radiant energy (e.g. infrared radiant energy), to absorb the energy and reach higher levels of molecular vibration and rotation, thereby to reach a higher temperature and pressure. When the radiant energy is amplitude modulated, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations. A sensitive microphone can be used to generate an electrical output representing the pressure fluctuations. The amplitudes of the acoustic signal and resulting electrical output are proportional to the intensity of the radiation and the concentration value of the absorbing gas. Accordingly, given a constant amplitude of radiant energy illumination, the electrical output can be detected at the modulating frequency to provide a concentration value proportional to an absorbing amount of the gas. Further, the proportional relationship with light source intensity allows the user to increase sensitivity by increasing light source intensity. Thus the devices are well suited for measuring small concentration values of gases (ppm, i.e., parts-per-million range), especially as compared to sensors that rely on measurement of transmitted radiant energy.
A variety of these devices are known, e.g. see U.S. Pat. No. 4,557,603 (Oehler et al), U.S. Pat. No. 4,818,882 (Nexo et al), and U.S. Pat. No. 4,866,681 (Fertig). The devices have several components in common. In particular, a laser or other energy source produces radiant energy which is modulated either thermally (power on/off) or with a chopping device. The modulated energy is provided to a cell containing a gas or gas mixture that absorbs the radiant energy, leading to temperature fluctuations in the gas that track the modulation frequency. Temperature is not sensed directly. Rather, pressure fluctuations that accompany the temperature fluctuations are detected by a sensitive microphone in the cell. The microphone output is detected at the modulation frequency, to provide an electrical signal proportional to the gas concentration.
Frequently there is a need to determine the concentrations of two or more gases within a gas mixture. While this could be accomplished with two or more sensing systems, one devoted to each of the gases under study, a sharing of components among several systems would likely reduce costs. Accordingly there have been several proposals involving use of a single photoacoustic cell to detect two or more gases.
For example, the Nexo et al patent discloses a perforated disk with three sets of filter openings with different spacings between adjacent openings, for simultaneously: (1) filtering infrared light into different wavelengths "absorbed by N.sub.2 O, CO.sub.2, and anesthetics, respectively"; and (2) modulating the wavelengths at three different frequencies. Signals corresponding to the various gases are said to be separated through an electric filtration of the microphone signal.
The Oehler et al patent discloses a mechanical light modulator and monochromator having different interference filters said to enable simultaneous separate detection of several components of a gas mixture. Oehler indicates that the interference with measurement by other gas components can be largely eliminated by using more than one narrow-band filter adapted to the maxima or flanks of the measuring gas or interfering components. Concentrations of different components are said to be determinable from the measurements performed with the different narrow band filters, with these filters being successively introduced into the path of the rays.
One disadvantage of these systems is the need to provide the radiant energy in extremely narrow bands. This requires either lasers for generating energy, or equipment designed to successively introduce different narrow-band filters into the light path between the source and photoacoustic cell. Either approach adds to the cost of the system. Further, it is difficult within the confines of these systems to distinguish between two gases with overlapping or coinciding absorption bands, or to determine the presence of an unknown absorbing gas.
Therefore, it is an object of the this invention to provide for the simultaneous sensing of two or more gases by: using shared components, e.g. a single photoacoustic cell, microphone and amplifier; avoiding the need to generate radiant energy solely as monochromatic beams; and using no moving parts.
Another object is to provide the capability of separately measuring the concentration values of several gases having absorption lines or bands which may overlap or coincide with one another, or detecting the presence of another gas whose absorption bands or lines may overlap or coincide with those of the several gases whose sensing is desired.
Another object is to render the sensor more reliable, by correcting concentration values for effects of varying temperature, atmospheric pressure, and elevations of installed sensors relative to sea level.